1. Field of the Invention
The present invention relates to a signal cable for transmitting electric signals, and in particular to a preferable technique when the signal cable is connected to an acoustic device or video device and is used as a high quality acoustic signal cable or video signal cable for increasing the transmission efficiency of acoustic signals or video signals.
2. Prior Arts
Conventionally, a two-strand coaxial cable equipped with a pair of signal lines and shielded by a shielding line provided at the circumference thereof via an insulating layer and a set of coaxial cable which shields a signal line with a shielding line via an insulating layer have been used as an acoustic cable or video cable making a connection between acoustic devices or video devices.
An acoustic cable or video cable is required to transmit signals in such states that the transmission signals are prevented from being attenuated and major frequency components of the signals to be transmitted are faithfully maintained, in order to increase the transmission efficiency of signals transmitted by the cable. Therefore, these kinds of signal cables are variously devised to decrease the attenuation and strain of a signal waveform and to eliminate influences due to external noise by improving the amplitude characteristics and phase characteristics of signals to be transmitted.
As one of these devices, for example, a balance type two-strand shield cable which is a set of coaxial cable consisting of a signal line and a shielding line to shield the signal line via an insulation layer is disclosed in Japanese patent publication No. Hei-3-17330, wherein the shield line connected for continuity to the inputting part of the signal line is prepared, and the set of cable is further shielded and formed by a common grounding shield.
Furthermore, another Japanese patent publication Hei-1-302612 discloses a cable in which a set of signal lines and shield line are connected to each other at the inputting side similarly to that in the above patent, the circumference thereof is shielded by another shield line, and these are further shielded by insulation resin.
These signal cables succeeded in decreasing the loss of transmission signals and influences of external noise to some degrees by further providing another shield line outside the shield line of the coaxial cable as shown above.
The above signal cables have been greatly improved with regard to both faithfulness of transmission signals and elimination of external noise, in comparison with prior conventional signal cables. However, it is necessary to shield signal lines by two conductor layers via three insulation layers, and furthermore as it is necessary to connect the respective two shield lines to the signal lines and an grounding end of a device in addition to the signal lines at the connection, the construction thereof is complicated to cause it to be very difficult to lower the production cost thereof.
Furthermore, there are some acoustic devices and video devices, which function improperly if a shield line of the signal cable is connected to the grounding line of the device. In a case where it is intended that the signal cable is connected to such devices, it is necessary to secure a separate protection circuit between the grounding line of the device and internal circuits. Still furthermore, some devices do not have any grounding line to which the shield line of signal cable is connected. In a case where the above signal cable is connected to such devices, it is necessary to provide grounding in addition to the device.